check wether an arraylist has a value

Question

I am trying to solve my problem where i have to look if when inputing a value into an arraylist this value already been added, and if so I want to display a message along the lines of name already exists

import java.util.*;

class Bank{

void command(){
 CommandReader commandReader=new CommandReader(this);
 commandReader.run();
}
ArrayList <Account> accounts=new ArrayList <Account>();

public static void main(String[] args){
 new Bank().command();

}
void print(String name){

System.out.println("Account of "+name);
int b=0;
for(int i=0; i<accounts.size(); i++){ 
  b=accounts.get(i).balance(name);
  break;
  } 
System.out.println("Balance: "+b);//dit moet naar account verwjizen op een of andere      manier zodat hij weet waar hij balance() moet zoeken
 }
void printAll(){

/*
for(int i =0; i<accounts.size()-1;i++){
 print(accounts[i].name);//klopt niet, weet niet wat er wel moet staan
}*/
}
void enroll(String name){
Account account=new Account(name,0);
accounts.add(account);
}
void deposit(String name, int amount){
for(int i=0; i<accounts.size(); i++){
  accounts.get(i).deposit(amount, name);
  } 
}


 void withdraw(String name, int amount){//hiervoor moet je denk ik een methode in account maken zodat e met balance kan rekenen?
for(int i=0; i<accounts.size(); i++){
  int x;
  x=accounts.get(i).balance(name);
  if (x>-1000){
  amount = amount*-1;
   accounts.get(i).deposit(amount, name);

  } 
}
}
  void printRed(){

}
void interest(double rate){
for(int i=0; i<accounts.size(); i++){
  accounts.get(i).inter(rate);
}
 }   

}

class Account{
String name;
int balance;

void inter(double rate){
 balance+=rate
   /100*balance;
}

Account(String n, int b){//weet niet of dit nodig is, denk het wel
 this.name=n;
 this.balance=b;
}
String name(){
  return this.name;      
} 
int balance(String name){
  if(name.equals(this.name)){
    return this.balance;
  }
  return 0;
}
void deposit(int b, String name){
  if(name.equals(this.name)){
  this.balance+=b;
  }
}
 }

is my way to add values into the array via methods.(the zero can be ignored)

Answer 1

You can do it in two ways:

1. Write a comparison yourself. Then go through all the array elements to verify if the object already exists.

2. Let your Account class override equals and hashCode methods, then use List#contains to verify if the object is already present in the list.

   A short example of this:

   Account account = new Account(name,0);
   //this assumes that Account class has already overridden equals and hashCode methods
   if (!accounts.contains(account)) {
       accounts.add(account);
   }
   

If you're going to use the latter, it would be better using a Set instead of a List since Sets already handle that the object should be unique in the collection.

---

From your edit, the current error is that Account class must override equals and hashCode methods. When you do this, the code will behave as expected.

Providing a basic implementation for both:

class Account {
    String name;
    int balance;

    void inter(double rate) {
        balance+=rate
       /100*balance;
    }

    Account(String n, int b) {
        this.name=n;
        this.balance=b;
    }
    String name() {
        return this.name;
    }
    int balance(String name) {
        if(name.equals(this.name)) {
            return this.balance;
        }
        return 0;
    }
    void deposit(int b, String name){
        if(name.equals(this.name)){
            this.balance+=b;
        }
    }
    //from here, this is the code you need
    @Override
    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (o instanceof Account) {
            Account another = (Account)o;
            return this.name.equals(another.name);
        }
        return false;
    }
    @Override
    public int hashCode() {
        //if working with Java 7, uncomment below LoC
        //return java.util.Objects.hash(name, balance);
        //if using Java SE 6 or less
        return java.util.Arrays.hashCode(new Object[]{ name, balance });
    }
}

Answer 2

override equals() and hashcode(), Use Set instead and its add() returns boolean depending on if it has ignored